Solvent-free selective hydrogenation of nitroaromatics to azoxy compounds over Co single atoms decorated on Nb2O5 nanomeshes

The solvent-free selective hydrogenation of nitroaromatics to azoxy compounds is highly important, yet challenging. Herein, we report an efficient strategy to construct individually dispersed Co atoms decorated on niobium pentaoxide nanomeshes with unique geometric and electronic properties. The use of this supported Co single atom catalysts in the selective hydrogenation of nitrobenzene to azoxybenzene results in high catalytic activity and selectivity, with 99% selectivity and 99% conversion within 0.5 h. Remarkably, it delivers an exceptionally high turnover frequency of 40377 h–1, which is amongst similar state-of-the-art catalysts. In addition, it demonstrates remarkable recyclability, reaction scalability, and wide substrate scope. Density functional theory calculations reveal that the catalytic activity and selectivity are significantly promoted by the unique electronic properties and strong electronic metal-support interaction in Co1/Nb2O5. The absence of precious metals, toxic solvents, and reagents makes this catalyst more appealing for synthesizing azoxy compounds from nitroaromatics. Our findings suggest the great potential of this strategy to access single atom catalysts with boosted activity and selectivity, thus offering blueprints for the design of nanomaterials for organocatalysis.


Synthesis of Co1/MgO
The synthetic method of MgO was based on a previously reported study 1 .In a typical synthesis, MgO nanosheets with highly exposed (111) facets were prepared with hydrothermal method: 2.0 g of MgCl2 and 0.12 g of benzoic acid were dissolved in 60 ml of deionized water under sonication and then stirred for 15 min.20 ml of 2 M NaOH solution was added into the mixture with vigorous stirring.The slurry was subsequently transferred to a 100 ml of autoclave and heated at 180 ºC for 24 h.The obtained Mg(OH)2 precursor was washed and dried at 80 ºC.Finally, the MgO(111) nanosheets were obtained following calcination of the Mg(OH)2 precursor in air at 500 ºC.The preparation method for Co1/MgO was the same as that of Co1/Nb2O5.

Synthesis of Co1/V2O5
The synthetic method of V2O5 was based on a previously reported study 2 .In a typical synthesis, 2 g of ammonium metavanadate and 4.5 g of oxalic acid were dispersed in 20 ml of deionized water.Then, the solution was dried by a rotary evaporator and the obtained powder was calcined at 300 ºC for 4 h to obtain yellow V2O5.The preparation method for Co1/V2O5 was the same as that of Co1/Nb2O5.

Synthesis of Co1/N-C
In a typical synthesis, 0.558 g of Zn(NO3)2•6H2O was dissolved in 15 ml of methanol and subsequently added to 15 ml of methanol with 0.616 g of 2-methylimidazole under sonication.The mixture was left static at room temperature for 12 h before the precipitates were washed and vacuum dried at 80 ºC.The as-prepared ZIF-8 powder was grounded and heated at 950 ºC for 2 h with a ramping rate of 5 ºC min -1 to give Ndoped carbon (N-C).The preparation method for Co1/N-C was the same as that of Co1/Nb2O5.

Characterization
X-ray diffraction (XRD) patterns were obtained by using a SmartLab SE diffractometer with Cu Kα radiation.Scanning electron microscopy (SEM) images were taken by an FEI Quanta450 microscope.Transmission electron microscopy (TEM) images were recorded on an FEI Tecnai G2 F20 microscope.High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images were collected by a JEOL JEM-ARM200F TEM/STEM with a spherical aberration corrector.X-ray photoelectron spectroscopy (XPS) measurements were carried out on a Thermo ESCA Lab 250XI spectrometer with a monochromated Al K a radiation (1486.6 eV) source.N2 sorption analysis were conducted with an ASAP 2460 instrument.Raman spectra were recorded using a Horiba HR Evolution Raman spectrometer with a 532 nm laser.
CO chemisorption, H2-temperature-programmed reduction (H2-TPR), and H2temperature-programmed desorption (H2-TPD) were performed on a Micromeritics AutoChem II 2920 apparatus.Electron paramagnetic resonance (EPR) spectra were collected with a Bruker EMX plus model spectrometer.Fourier transform infrared (FT-IR) spectroscopy was performed with a Nicolet IS5 spectrometer.In situ CO-DRIFTS were recorded on a Nicolet IS50 FT-IR spectrometer equipped with a liquid nitrogencooling mercury-cadmium-telluride (MCT) detector.Before the experiment, the catalyst was pretreated under a high-purity N2 atmosphere at 130 º C for 30 min.After cooling down to room temperature, the background was collected.For the CO adsorption, high-purity CO gas (50 mL/min) was fed into the cell for 30 min up to adsorption saturation.For the CO desorption, the high-purity N2 flow (30 mL/min) was purged into the cell to remove adsorbed CO.For the nitrobenzene hydrogenation reaction, the procedures were as follows: first, the catalyst was placed in an in-situ reaction cell and pretreated with high purity N2 flow (30 mL/min) at 130 º C for 1 h.
After cooling down to room temperature, the mixture of gasified nitrobenzene and highpurity N2 flow (30 mL/min) was introduced into the reaction cell for 30 min, and then the gas flow was changed to H2/N2 flow (v: v = 5: 95; 30 mL/min) for nitrobenzene hydrogenation.The metal loadings were analyzed with a PerkinElmer Optima 7300 DV.
Thermogravimetric analysis (TGA) was performed on a Setaram Setline STA thermogravimetric analyzer.H2-D2 isotopic exchange experiments were conducted on a chemisorption analyzer (Micrometritics, AutoChem II 2920).Specifically, 50 mg of the catalyst was pre-treated in H2 atmosphere (10 mL/min) at 200 ºC for 1 h and cooled down to 50 ºC.Subsequently, the gas mixture of H2 (20 mL/min) and D2 (20 mL/min) was fed into the reactor and the catalyst was heated at a rate of 10 ºC/min.The signals of H2 (m/z=2), D2 (m/z=4), and HD (m/z=3) were collected by a mass spectrometer (GSD350 OmniStar) and normalized by the mass of catalyst.X-ray absorption fine structure (XAFS) at Co K-edge of samples were collected in transmission mode at 1W1B beamline of the Beijing Synchrotron Radiation Facility.The EXAFS data were processed using the ATHENA module implemented in the IFEFFIT software packages.
To obtain the quantitative structural parameters around central atoms, least squares curve parameter fitting was performed using the ARTEMIS module of IFEFFIT software packages.X-ray absorption near-edge structure (XANES, Co L-edge and O K-edge) measurements were conducted at the Beamline of MCD-A and MCD-B (Soochow Beamline for Energy Materials) and the ultraviolet photoemission spectroscopy (UPS) measurements were performed at the Beamline of BL11U of National Synchrotron Radiation Laboratory in Hefei, China.The valence band spectra were collected with a photon energy of 40 eV and referenced to the Fermi level determined by an Au foil.

Computational methods.
All calculations were performed by using the density functional theory (DFT), as implemented in the Vienna ab-initio simulation package (VASP) 3,4 .The exchangecorrelation potential was treated by generalized gradient approximation (GGA) in the form of Perdew-Burke-Ernzerhof (PBE), and the energy cutoff was set to 400 eV 5,6 .All structures were relaxed until a residual force was less than 0.05 eV Å −1 .The energy convergence criterion was set to 5 × 10 −5 eV.The Brillouin zone integration was conducted by using a 2 × 2 × 1 k-point mesh for the calculations of electronic structures and reaction energies.The transition state (TS) was determined using the double-ended surface walking (DESW) and constrained Broyden dimer (CBD) approaches, as implemented in the LASP software 7,8 ; the methods can establish a low-energy pathway linking two minima even without iterative optimization of the pathway, from which the TS can be located readily.All transition states were verified by vibrational frequency calculations (only one imaginary frequency).The solvent effect was considered using the Poissson-Boltzmann implicit solvation model 9 , in which the dielectric constant ε was taken as 18.36 for THF: H2O (v: v = 4: 1) as a demonstration 10 .The DFT-D3 method is adopted to correct van der Waals interactions 11 .A vacuum layer in the zdirection was set to over 20 Å.To minimize the self-interaction error, the Hubbard U correction was applied to the 3d orbitals of Co, where the Ueff value was set to 3.4 eV 12 .

Table 2 .
Structural parameters of samples extracted from the EXAFS fitting with reference of Co foil at the Co K-edge.(Ѕ0 2 =0.82) N: coordination numbers; R: bond distance; σ 2 : Debye-Waller factors; ΔE0: the inner potential correction.R factor: goodness of fit.Supplementary

Table 5 .
Catalytic performance of different catalysts in the selective hydrogenation of nitrobenzene to azoxybenzene with solvent.